There are numerous methods for performing seismic prospecting in formations crossed by a well, including the emission of seismic pulse signals or of vibrational signals by a source lowered into the well, in order notably to avoid the disturbances transmitted by the weathering zone when the signals are emitted at the surface, the reception of the signals reflected by the subsoil discontinuities by means of a reception device located in the well, in another well or at the surface, and the recording thereof. Numerous processings are generally applied to the recordings so as to make them more readable to the interpreter.
Among the numerous emission devices for wells, some have been specially designed to work in relation or in combination with a drill bit. These operations, referred to as MWD operations, allow prospecting operations to De achieved without stopping the drilling of a well and therefore exploration costs to be decreased.
Emission devices are for example known with which the vibrations applied to formations are the "noises" generated by a drill bit operated at the end of a drillstem or drill string. The amplitude of these vibrations may vary to a large extent according to the hardness of the formations crossed by the bit. Conventional processing methods are generally very difficult to utilize with the recordings obtained with this type of emission because the characteristics of the source are not well-known. The signal is emitted permanently and no reference instant can be set. Moreover, the frequency spectrum emitted depends on the rotating speed of the bit and on the nature of the beds drilled. In order to know at any time the phase of the signals emitted, it is necessary to have a sensor located close to the drill bit and any transmission channel allowing a signal in phase with the signals emitted to be transmitted to a surface processing equipment. It may be, for example, the transmission of acoustic waves modulated according to the signals to be transmitted, along the fluid contained in the well, as described for example in U.S. Pat. No. 4,675,852, or possibly along the pipes of a drill string.
Among well sources, those which transmit, to the formations crossed, seismic impulses generated by shocks of the drill bit on the hole bottom may also be cited, these shocks being isolated or repeated with a determined period. To create these shocks, it is possible to use the surface operating device to exert a momentary traction on the drill string and to vary momentarily the force of gravity applied onto the bit. The shocks Of the drilling head on the hole bottom may also be generated by a percussion drill bit constituted by interposing, between the drilling head and the drill string, a telescopic hydraulic device which may be contracted through a momentary pressure on the string and which, while slackening with a given delay when this pressure is loosened, applies a shock onto the bit. It is also possible to bring about shocks on the drill bit by triggering off "water hammers" through a sudden interruption of the mud flow circulating within the drill string. The main drawback of devices of this type is that, as they are interposed between the drill bit and the drill string, they require modifications in the usual drilling equipment and they have to bear the total weight exerted on the bit, which may be a source of brittleness.